As next-generation radio communication technology beyond 4G (Long Term Evolution (LTE)-Advanced), 5G has been attracting attention. Various studies on 5G including use cases and requirements for 5G have been started. Reduction in access latency (Latency Reduction) is drawing attention as a major requirement for 5G. Examples of access latency includes: a time required for radio terminal (User Equipment (UE)) to complete transmission of data to a radio base station (eNodeB (eNB)); a time required for a UE to complete transmission of data to an external network (e.g., application server); a time required for a UE to complete transmission of data to another UE; or a time required for a UE to establish preparation for transmitting data. In the present specification, a time period from when a radio terminal (UE) requests a grant for transmission of uplink (UL) data from a radio base station (eNB) until when the UE completes the transmission of the UL data to the radio base station (eNB) is used as an example of the access latency. Further, radio communication in which some kind of access latency is reduced as compared to existing radio communication (e.g., LTE and LTE-Advanced) is collectively referred to as “low latency access”.
Non Patent Literature 1 discloses a Time Division Duplex (TDD) system (hereinafter referred to as B4G TDD) where both uplink (UL) and downlink (DL) exist within one subframe in order to achieve low latency access. In B4G TDD, for example, a request for UL radio resources from an UE (i.e., Scheduling Request (SR)), transmission of allocation information of the UL radio resources from an eNB (i.e., Scheduling Grant (SG)) in response to the request, and transmission of UL data in accordance with the SG are performed within one subframe. If the UL data is successfully received by the eNB in one transmission, access latency (in this case, the access latency is assumed to be a time period from when the UE transmits the SR to the eNB until when the UE completes the transmission of the UL data) is reduced from about 10 millisecond (ms), which has been required before, to a subframe length (e.g., 1 ms). Non Patent Literature 1 also discloses using a subframe length (e.g., 0.25 ms) that is shorter than the subframe length (i.e., 1 ms) of LTE and LTE-Advanced. This contributes to further reduction of the access latency.